1. Field of the Invention
The present invention relates to a light guide, a line-illuminating device, and a contact-type or reduction-type image-scanning device (image sensor) in which the line-illuminating device is incorporated.
2. Description of the Background Art
An image-scanning device for a facsimile machine, a copying machine, an image scanner or the like is provided with a line-illuminating device for linearly illuminating a document surface along a main scanning field. In the line-illuminating device, light is introduced into an end surface of a bar-shaped or plate-shaped transparent light guide, reflected on the internal surfaces, and emitted from a light-emitting surface.
Since a light-emitting element such as an LED is provided on one end or both ends of the light guide of the line-illuminating device, the amount of emitted light is greater toward the end, and the light intensity is non-uniform in the main-scanning direction.
Thus, Patent Document 1 has proposed that a light scattering pattern is formed on a surface of the light guide by printing white pigment, and the width of the light scattering pattern is gradually decreased toward the light-emitting unit. Also, Patent Document 2 has proposed that a light scattering pattern formed on the end near to the light-emitting unit is a discontinuous shape.
However, since a light scattering pattern is formed by printing white pigment with a screen printing method in the above-mentioned cases, the magnitude of transfer dots is varied due to many factors such as clogging of the screen, the temperature, the humidity, the dilution degree of the solvent, or diffusion of the pigment by static electricity, whereby a preferable light scattering pattern cannot be reproduced and the production yield is deteriorated. If a silk-screen printing method is used, since correction and production need to be repeated until satisfactory uniformity is achieved, it takes a lot of time and money.
In order to solve the above-mentioned drawbacks, Patent Document 3 has proposed that a light-reflecting portion comprised of an irregular surface having a triangular shape is provided in the rear surface (bottom surface) of the light guide.
[Patent Document 1] Japanese Patent Application Publication No. 8-163320
[Patent Document 2] Japanese Patent Application Publication No. 10-126581
[Patent Document 3] Japanese Patent Application Publication No. 2001-242322
FIG. 12 is a view explaining light reflection in the case where an irregular surface having a triangular shape is formed as a light-reflecting portion. Light emitted from the light-emitting element such as an LED enters the inside of the light guide through the end surface of the light guide, and the light is reflected on the irregular surface so as to reach the light-emitting surface. If the light is incident on the light-emitting surface at a smaller angle than the total reflection angle, the light is transmitted through the light-emitting surface and emitted. If the light is incident on the light-emitting surface at a larger angle than the total reflection angle, the light returns back to the inside of the light guide.
In this instance, since the incident angle of light on the irregular surface is not uniform in the area close to the end where light enters, the reflection angle is not uniform either, whereby the intensity of emitted light is uniform to some extent. However, since the incident angle of light on the irregular surface becomes almost uniform in the area far from the end where light enters, the reflection angle also becomes almost uniform, whereby the intensity of emitted light becomes non-uniform.